An initial-boundary value problem for a time-fractional subdiffusion equation with an arbitrary order elliptic differential operator is considered. Uniqueness and existence of the classical solution of the posed problem are proved by the classical Fourier method. Sufficient conditions for the initial function and for the right-hand side of the equation are indicated, under which the corresponding Fourier series converge absolutely and uniformly. In the case of an initial-boundary value problem on N -dimensional torus, one can easily see that these conditions are not only sufficient, but also necessary.

An initial-boundary value problem for a time-fractional subdiffusion equation with the Riemann-Liouville derivatives on N-dimensional torus is considered. Uniqueness and existence of the classical solution of the posed problem are proved by the classical Fourier method. Sufficient conditions for the initial function and for the right-hand side of the equation are indicated, under which the corresponding Fourier series converge absolutely and uniformly. It should be noted, that the condition on the initial function found in this paper is less restrictive than the analogous condition in the case of an equation with derivatives in the sense of Caputo.

An initial-boundary value problem for a time-fractional subdiffusion equation with the Riemann-Liouville derivatives on N -dimensional torus is considered. Uniqueness and existence of the classical solution of the posed problem are proved by the classical Fourier method. Sufficient conditions for the initial function and for the right-hand side of the equation are indicated, under which the corresponding Fourier series converge absolutely and uniformly. It should be noted, that the condition on the initial function found in this paper is less restrictive than the analogous condition in the case of an equation with derivatives in the sense of Caputo.

An initial-boundary value problem for a time-fractional subdiffusion equation with an arbitrary order elliptic differential operator is considered. Uniqueness and existence of the classical solution of the posed problem are proved by the classical Fourier method. Sufficient conditions for the initial function and for the right-hand side of the equation are indicated, under which the corresponding Fourier series converge absolutely and uniformly. In the case of an initial-boundary value problem on N -dimensional torus, one can easily see that these conditions are not only sufficient, but also necessary.

We investigate an initial-boundary value problem for a timefractional subdiffusion equation with the Caputo derivatives on N -dimensional torus by the classical Fourier method. Since our solution is established on the eigenfunction expansion of elliptic operator, the method proposed in this article can be used to an arbitrary domain and an elliptic operator with variable coefficients. It should be noted that the conditions for the existence of a solution to the initial-boundary value problem found in the article cannot be weakened, and the article provides a corresponding example.